The invention relates to the manufacturing of electric cables in general and more particular to the SZ twisting of power cable conductors of sector-shaped cross section with special consideration of the mechanical forces occurring in this process.
To better utilize the space in the cable cross section, it is customary in multi-conductor power cables in the low and medium voltage range (up to 10 kV), to use cable conductors of sector shaped cross section. These are twisted together with constant direction of twist to form the cable core. Sector conductors with or without initial twist can be used. Sector conductors without a pre-twist are twisted together without back twisting; strong torsional stresses then act on the sector conductors during the twisting process. Sector conductors with pre-twist are twisted together with back twisting; then, the torsional stresses are relatively low ("Kabel-und Leitungsfertigung", Kombinat VEB Kabelwerk Oberspree, VEB-Verlag Technik Berlin, 1976, page 200). In twisting sector conductors, it is customary to arrange a positive guide ahead of the twisting closer in order to fix their position in space required for the twisting. Such a positive guide can consist of several profiled rollers which are arranged one behind the other in a straight line and between which the respective conductor runs (DE-OS No. 22 11 111).
Besides the customary twisting of the sector conductors with constant twist direction, a type of twisting is also known, in which the sector conductors are twisted with a twist direction which changes at intervals. Such a type of twisting has found acceptance in recent years under the designation "SZ twisting" in the manufacture of communication cables and power lines, but the SZ twisting machines developed for this purpose cannot be employed without appropriate further development for the twisting of sector conductors, as large mechanical forces must be controlled in the twisting of sector conductors because of the large conductor cross section (more than 35 mm.sup.2).
In one heretofore known apparatus for the SZ twisting of sector conductors without a pre-twist, there is provided as the twisting tool proper an oscillating twisting disc arranged ahead of the twisting closer, which is coupled to positive guides for the sector conductors. With such a twisting equipment, naturally only one to two lengths of lay of twist can be generated for each direction of twist (DE-OS No. 25 14 033). It has further been proposed already to twist the sector conductors straightened ahed of the twisting closer by means of a twisting device arranged between a first and a second twisting closer. This device revolves with a direction of rotation changing at intervals, so that the sector conductors are twisted a first time in the first twisting closer and a second time in the same direction in the second twisting closer. The twisting closer consists of one or two collet pull-off devices arranged one behind the other. Optionally, non-rotating collet pull-off devices can be arranged between the twisting points of the twisting machine, in order to always ensure equal distances between the stationary and rotating parts of the twisting machines which are responsible for the twisting, for the purpose of obtaining constant length of lay. The direction of rotation of the twisting device is changed at intervals which are matched in a manner known per se to the storage capacity if the device (British Pat. No. 2,004,575).
For SZ twisting round line conductors of larger cross section, an SZ twisting machine is also known which contains two twisting tools, namely, an oscillating hole plate and an oscillating rotating twisting head in the form of a caterpillar pull or a three-pulley arrangement. While the hole plate is arranged in front of a first twisting closer, and the twisting head is located between the first and the second twisting closer (DE-OS No. 24 12 199).
There is further known, for SZ twisting conductors for communication cables, a twisting method in which the elements to be twisted are stranded by means of a stretched accumulator of varying rotary motion. The stretched accumulator consists of two twisting heads arranged between two twisting closers. The direction of rotation or the speed of rotation of the twisting heads is changed at intervals which are matched to the storage length of the accumulator (British Pat. No. 1,095,434; U.S. Pat. No. 3,823,536). For such twisting purposes, twisting heads can also be used which are designed in the manner of a caterpillar pull-off device (DE-OS No. 17 90 249) or consist of a deflection pulley over which the material to be twisted is looped once and is arranged tangentially to the twisting axis (U.S. Pat. No. 3,593,509).